Capture modes

The DCMI supports two types of capture: snapshot (a single frame) and continuous grab (a sequence of frames).

The user can control the capture rate by selecting the bytes, lines, and frames to capture in the DCMI_CR register. These features are used to convert the color format of the image, and/or to reduce the image resolution (by capturing one line out of two, the vertical resolution is divided by 2). For more details, refer to Image resizing (resolution modification).

In this mode, a single frame is captured. After the capture is enabled by setting the CAPTURE bit in DCMI_CR, the interface waits for the detection of a start of frame (the next DCMI_VSYNC or the next embedded frame-start code, depending on the synchronization mode) before sampling the data.

Once the first complete frame is received, the DCMI is automatically disabled (CAPTURE bit automatically cleared), and all the other frames are ignored. In case of an overrun, the frame is lost and the camera interface is disabled.

Once this mode is selected and the capture is enabled (CAPTURE = 1), the interface waits for the detection of a start of frame (the next DCMI_VSYNC or the next embedded frame-start code, depending on the synchronization mode) before sampling the data.

In this mode, the DCMI can be configured to capture all the frames, every alternate frame (50% bandwidth reduction), or one frame out of four (75% bandwidth reduction). The camera interface is not automatically disabled but the user must disable it by setting CAPTURE = 0 . After being disabled by the user, the DCMI continues to grab data until the end of the current frame.

For monochrome, RGB and YCbCr, the frame buffer is stored in raster mode as shown in Figure 30 Pixel raster scan order.

Data received from the camera can be organized in lines, frames (raw YUV/RGB/Bayer modes), or can be a sequence of JPEG images.

The number of bytes in a line may not be a multiple of four. The user must therefore be careful when handling this case since a DMA request is generated each time a complete 32-bit word has been constructed from the captured data. When an end of frame is detected and the 32-bit word to be transferred has not been completely received, the remaining data are padded with zeros, and a DMA request is generated.

RGB refers to Red, Green, and Blue, which represent the three hues of light. Any color is obtained by mixing these three colors.

Each component has the same spatial resolution (4:4:4 format): each sample has a red (R), a green (G) and a blue (B) component. Figure 32 DCMI data register filled with RGB data shows the DCMI data register containing RGB data, when an 8-bit data width is selected.

YCbCr is a family of color spaces that separates the luminance or luma (brightness) from the chrominance or chroma (color differences).

YCbCr 4:2:2 is a subsampling scheme, which requires a half resolution in horizontal direction: for every two horizontal Y samples, there is one Cb or Cr sample.

Each component (Y, Cb, and Cr) is encoded in 8 bits. Figure 33 DCMI data register filled with YCbCr data shows the DCMI data register containing YCbCr data when an 8-bit data width is selected.

For compressed data (JPEG), the DCMI supports only the hardware synchronization, and the input size is not limited. Each JPEG stream is divided into packets, which have programmable size. The packet dispatching depends on the image content, and results in a variable blanking duration between two packets.

To allow JPEG image reception, the JPEG bit must be set to one in the DCMI_CR register. The JPEG images are not stored as lines and frames. The DCMI_VSYNC signal is used to start the capture while DCMI_HSYNC serves as a data enable signal.

If the full data stream finishes and the detection of an end of stream does not occur (DCMI_VSYNC does not change), the DCMI pads out the end of the frame by inserting zeros: if the stream size is not a multiple of four, at the end of the stream, the DCMI pads the remaining data with zeros.

Note:

The crop feature and embedded synchronization mode cannot be used in the JPEG format.

With the crop feature, the camera interface selects a rectangular window from the received image. The start coordinates (upper-left corner) are specified in the 32-bit DCMI_CWSTRT register.

The window size is specified in number of pixel clocks (horizontal dimension), and in number of lines (vertical dimension) in the DCMI_CWSIZE register.

As described in Capture modes, the DCMI capture features are set through the DCMI_CR register.

The DCMI captures all received lines, or one line out of two (the user can choose to capture the odd or even lines).

This feature affects the vertical resolution that can be received by the DCMI as sent from the camera module or divided by two (only the odd or the even lines are received).

Figure 36 Frame resolution modification shows one frame when receiving only one byte out of four and one line out of two.

All interrupts can be masked by software. The global interrupt dcmi_it is the logic OR of all the individual interrupts.

If an event occurs and the corresponding interrupt is enabled, the DCMI global interrupt is generated.